1. Field of the Invention
The invention relates to coatings, in particular, coatings comprising siloxanes.
2. Discussion of the Related Art
Glass optical fibers drawn from a heated glass preform or glass melt exhibit good strength as long as the surface of the drawn fiber remains defect-free. Thus, protective coatings, typically formed from thermoplastics or liquid-applied curable polymer-based resins, are normally applied to fibers immediately after they are drawn and prior to contact between the fiber and any other surface. Typically, the fiber is passed through a fluid polymer that is diluted in monomer or in a solvent to effect the coating, e.g., by use of a draw cup, such as that disclosed in U.S. Pat. No. 4,264,649. Optical fibers used in applications where temperatures are high, e.g., exceeding 175.degree. C., for extended periods of time require a coating that is mechanically stable at such temperatures, i.e., a coating with high thermal mechanical stability. Conventional ultraviolet-cured acrylate coatings can be safely used only at temperatures below about 100.degree. C. For higher temperatures, solvent-based polyimide coatings are often used. However, due to low polymer solubility, only thin coatings, e.g., much less than 10 .mu.m, can be applied at each pass of the fiber through a coating composition. Since a protective coating about 10 to about 50 .mu.m thick is generally desirable, several passes, e.g., several successive draw cups, are required to obtain a polyimide coating of adequate thickness. Moreover, the draw speed for solution-based polyimide coatings is limited to about 0.1 m/sec to allow for removal of solvent and completion of the imidization reaction, if required, before the coated fiber is able to be wound on a spool. Such limitations are not compatible with commercial requirements because slow draw speeds and a need for numerous passes through a draw cup complicate the process, reduce productivity, and increase expense.
Materials having siloxane backbones, e.g., crosslinkable primarily poly(dimethyl silicones), have been used for coating optical fibers for higher temperature applications. Silicone coatings, however, are rubbery, do not adhere well to glass fiber, and have poor mechanical strength. Organosilsequioxanes, which are precursors of siloxane ladder polymers (dual siloxane chains having cross-links that resemble the rungs of a ladder), have also been used for higher temperature coatings. See, for example, U.S. Pat. No. 5,052,779 to Honjo et al. and U.S. Pat. No. 4,835,057 to Bagley et al. Organosilsequioxanes are made by carefully controlled hydrolysis and condensation of organotrialkoxysilanes, typically triethoxysilanes. The resulting silanol groups condense with other silanol groups or with residual silyl ethoxy groups to form a siloxane bond, with the elimination of either water or alcohol. The condensation is typically accelerated by heat, e.g. temperatures of about 100 to about 150.degree. C., and by the addition of an acid or a base.
As reflected in the Honjo et al. patent, siloxane ladder materials are normally applied to a fiber as a solution of a solid silsequioxane polymer (known in the art as flake), having a molecular weight of about 5,000 to about 10,000, in a solvent, and thermally cured after solvent evaporation. Use of a solvent with the silsequioxane is also discussed in the Bagley et al. patent. Methoxy or ethoxy substituted silanes are normally used as precursors to the silsequioxanes, with the former being more reactive. Pendant groups on the siloxane backbone are often methyl or phenyl. Silsequioxanes form tougher and more adherent coatings than silicones, and generally have desirable thermal mechanical stability. However, the silsequioxane coatings described above require slow draw speeds, e.g., less than 0.5 m/sec, to obtain an acceptable, bubble-free, non-tacky coating. In the Bagley et al. patent, for example, the draw speed was 20 m/minute (about 0.3 m/sec) to obtain a relatively thin, 12 .mu.m coating. As stated above, such draw speeds are too slow for commercial applicability. Typically, for specialty, high temperature optical fiber, draw speeds of about 1 m/sec or higher are considered to be acceptable.
Thus, siloxane-based coatings which exhibit high thermal stability, i.e., mechanical stability at high temperatures, and which are capable of being applied at acceptable draw speeds, are desired.